A flexible pipe is made from a composite pipe structure with cross-sectional layers of polymers and steel that can accommodate large curvatures. Flexible pipe is used in a variety of applications including topside jumpers, flow-lines and, most importantly, risers.
Conduits to transfer materials from the seafloor to production and drilling facilities at the surface, as well as from the facility to the seafloor, are commonly termed risers. Risers are a type of pipe developed for this type of vertical transportation. Risers can serve as production or import/export means and are the connection between the subsea field developments and production and drilling facilities. Similar to pipelines or flow lines, risers transport produced hydrocarbons, as well as production materials, such as injection fluids, control fluids and gas lift. Risers can be made from steel pipes or flexible pipes.
Flexible risers made from flexible pipe can sustain large vertical and horizontal movement of the top end, making them ideal for use with floating facilities.
In general, a flexible pipe comprises a central steel carcass covered by a pressure sheath providing hydraulic integrity. Numerous layers of flexible armour surround the pressure sheath, or pressure vault, to provide tensile- and hoop-stress strength. The armour layers are usually separated by cushioning layers of composite or thermoplastic material to prevent them from rubbing against one another. The number and size of armour layers are a function of the pressure and tensile strength specifications imposed by the particular application for which the riser is designed. A final thermoplastic outer sheath provides protection towards the external environment.
A problem with some types of flexible pipes are that the internal carcass has a profile that can cause flow induced vibration or flow induced pulsations (FIP), commonly termed “singing”, inside the pipe.
This problem has been observed on floating production platforms that export or inject gas through flexible risers. As the gas flow is increased, high levels of distinctive tonal noise and vibration occur in the flexible pipe. These pulsations are generated by lock-in of synchronized vortex shedding in the groves on the inner corrugated layer (carcass) of the flexible pipe. When the vortex shedding frequency is close to the natural frequencies of topside piping or subsea manifolds, resonance can cause excessive vibration and noise levels in the topside and subsea piping. On-set of vibrations happen for gas flow velocities as low as 1.5 m/s. As the flow rate is limited by the first onset velocity, production capacity may be severely limited. This problem is discussed in, for instance, “Internal Flow Induced Pulsation of Flexible Risers” by R. Swindell, Bureau Veritas, and S. Belfroid, TNO TPD; 2007 Offshore Technology Conference, Houston. Tex., U.S.A.
Such vibrations have been suppressed by controlling the velocity of the gas, using smooth bore pipes or introducing silencers at the topside.
However, such solutions have drawbacks. Limiting the gas velocity will inherently limit the capacity of the pipe. Limiting the capacity of the gas export pipes has economic consequences and combined with limitation in gas injection pipes may result in reduced production capacity on the facility. Smooth bore pipes with a large inner diameter need special manufacturing techniques and have a more complex cross sectional structure, in which extra layers in the wall increase the wall thickness and limit the maximum diameter that can be produced.
An example of a riser comprising an inner layer or liner is known from U.S. Pat. No. 7,318,454, which describes a flexible tubular pipe for transporting fluids, particularly gaseous hydrocarbons, the pipe being of the non-bonded type and comprising at least a carcass, a polymer internal sealing sheath providing sealing for the transported fluid and one or more armour layers and in which the carcass, situated inside the internal sealing sheath consists of the interlocked spiral winding of a profiled element. The turns of the carcass are internally covered with a sheath pierced with holes that is intended to oppose turbulence of the fluid flowing in the pipe.
A problem with this solution is that the continuous inner sheath will impede the movement of the carcass and reduce the flexibility of the flexible riser, as it counteracts free relative movement of the interlocked carcass segments.
Silencers are large and heavy structures that need careful design to work and may have limited fatigue life. There is a significant risk that the silencer will not work properly for a given pipe. Silencers are costly and need a significant topside capacity.
Hence, there is a need for an arrangement that avoids the above problems. The object of the invention is to provide an improved flexible pipe carcass arrangement that will eliminate, or at least considerably reduce problems relating to flow induced vibration.